The tongue muscles participate in such diverse activities as breathing, swallowing, speech and mastication, and are thus critical for homeostasis. Fibers from eight different muscles insert into the mammaliantongue and control its movement, shape and stiffness, but the control of tongue muscle motor units has been largely ignored. Our goal is to explore how the central nervous system controls a patterned behavior (drive controlled by the respiratory central pattern generator, CPG) that involves multiple muscles acting on a single mechanical structure, by addressing the following issues: 1) motoneurons driving tongue protrudor and retractor muscles receive significant common synaptic input even though they have opposite mechanical actions on the tongue, suggesting that agonist-antagonist co-activation controlstongue stiffness; 2) respiratory-related input to tongue and "primary" inspiratory muscles (diaphragm, intercostals) is derived from independent sources; 3) Models predict that motor unit spike trains become more variable as synaptic input to the cell is increased. This can be tested by measuring the change in spike train variability when excitatory synaptic input is superimposed on the underlying input emanating from the respiratory CPG; 4} Motor units innervating tongue muscles with respiratory related activity fall into two functional populations; those that rate code when drive to the muscle increases, and those that do not. Wepropose that the firing rate of motor units that do not rate code saturates despite increases in synaptic input; and we will test this hypothesis; 5) Individual motor units within a muscle comprise at least four, task-specific sub- populations (inspiratory, expiratory, tonic and expiratory-inspiratory units), and the task specificity of a given unit depends on its contractile properties. Experiments will be done in anesthetized, spontaneously breathing adult rats. Techniques used include single motor unit electrophysiology, cross correlation analysis and the measurement of ventilatory output. The results of these experimentswill contribute to the knowledge base needed to develop treatment strategies for obstructive sleep apnea, swallowingdisorders, and oro-facial motor deficits.